The present invention relates generally to a system for cutting pieces having selected diverse lengths from a traveling strip of sheet material, such as plastic sheet, glass sheet, and laminated plywood sheet, and more particularly relates to a cutting system adapted for use in cutting a belt-like plastic sheet continuously cast between opposing traveling spans of long endless metal loops.
In various industries sheets which must be of different lengths to meet specifications, are obtained by first producing the sheet material such as plastic, glass and the like in strip form and then transversely severing the strip at intervals to provide the required sheets. Also, it is quite usual that the production of the sheet material and the severing of the required sheets are carried out in directly connected continuous processes in order to effectively use a limited factory space. That is, the severing of the required sheets is carried out without stopping the traveling of the sheet material continuously fed from the sheet material production process.
For this purpose, it has been customary to use an apparatus comprising a strip cutter and means for reciprocating this cutter through a cutting cycle composed of a cutting stroke and a return stroke. The cutting stroke moves the cutter diagonally across the traveling strip at an angle and speed which causes the cutter to sever the strip at right angles, and the return stroke returns the cutter to a position of readiness for a succeeding cutting cycle.
In the case of glass, the cutter may be a diamond, while, for example, rotary saws may be used as the cutter for plastics. Other examples would be a pressurized water jet or a laser ray, used as the cutter for both glass and plastic.
In one form of such an apparatus, the cutter is mounted on a carriage running on a track positioned diagonally and transversely over the traveling strip. The carriage is driven, for example, by sprocket chains which are driven by sprocket wheels, or by being in screw-threaded engagement with a threaded drive shaft extending along the length of the track. The design is such that the cutter carriage is driven through a cutting stroke, at a speed related to the angularity of the track, so that the cutter cuts the traveling strip at right angles. The cutter carriage drive is then reversed to effect a return stroke of the carriage, during which the cutter moves to an inoperative position free from the strip in readiness for a succeeding cutting stroke. In this fashion, the cutter is reciprocated through a cycle of the two strokes and the time for completion of this cycle, as related to the length of strip passing during this time, determines the minimum length of piece that can be severed from the traveling strip.
During both the cutting and return strokes the strip continues to travel, the speed of the cutting stroke being fixed as required by the angularity of the cutting stroke, and the strip traveling speed, and the speed of the return stroke being limited by practical mechanical design considerations.
In another type, the cutter, formed by a set of upper and lower rotary saws, is carried by a carriage which reciprocates at right angles to the traveling strip but with this carriage mounted on a frame to reciprocate parallel with respect to the traveling strip; with the frame moving synchronously with the strip while the cutter carriage moves transversely with respect to the strip, and with proper timing of the motions, the strip is severed at right angles. In this case the cutter carriage is ordinarily reciprocated transversely with respect to the frame by being in screw-threaded engagement with a threaded drive shaft. In any event, the return stroke of the cutter carriage cannot be made extremely fast. Therefore, with this type of apparatus, also, the minimum length of a piece that can be severed from a traveling strip, is limited.
As the production rate of each commercial manufacturing process has increased, the traveling speed of such strip material has also increased, making it necessary to sever from the strip, pieces which are shorter than the minimum length that can be cut by apparatus of the type described.
One example is in the production of a strip of cast plastic leaving a continuous casting machine of the type using two stainless steel endless belts defining a continuously traveling casting space into one end of which a liquid which hardens with temperature, is cast to produce the continuously traveling strip from its other end and requiring severing into commercially desirable lengths. One of these belts is usually longer than the other, and in each instance the endless stainless steel belt is formed by welding together the ends of a suitable length of stainless steel strip. The weld lines may mark the cast strip surfaces and such markings occur at random on the opposite surfaces of the strip. The marked portions must be removed by cutting and in the interests of manufacturing cost reduction, the marked pieces should be cut as short as possible, as they are mostly discarded as waste or used as reclaiming material.
In this instance, the type of apparatus using a set of upper and lower rotary saws, is more appropriate, but this apparatus cannot make repeated cuts rapidly enough to cut out the desired minimum lengths from the strip to remove the imperfect portions with a minimum of waste.
Also, with such continuously traveling cast plastic strip, which is of infinite length because of the continuous production involved, it may be desirable to produce two or more different lengths of commercially acceptable sheets from between the pieces which must be cut out and wasted because of being marked by the belt welds. The plastic strip may unfortunately require removal of imperfections or manufacturing defects other than the unavoidable belt weld marks. If the traveling strip could be cut rapidly enough, the imperfect portions could be removed and these could be of relatively short lengths, while the two or more different lengths could be severed from each usable portion between neighboring imperfect portions in a manner determined to produce an appropriate number of commercially acceptable sheets of the two or more different lengths while minimizing the loss which is defined as the remaining sheet portion resulting from severing the appropriate number of sheets of the two or more different lengths from the usable sheet portions. In this connection, the yield of sheets usable as first-grade products which can be obtained from the strip portions between the defects, as related to the amount of the cast plastic strip material which must be considered waste, has, of course, a direct relationship on the overall costs of manufacturing the plastic sheets in different commercial lengths, or lengths specified by customers of the sheet manufacturer.
Therefore, a principal object of the present invention is to provide a system for cutting or severing such strip sheet material, particularly, but not exclusively, continuously cast plastic strip sheet material, into sheets of commercially required lengths so as to obtain a better yield of sheets from the strip sheet material of infinite length containing the randomly interspaced defects.
Another object of the present invention is to provide a system capable of meeting the requirements of rapid cutting of the traveling strip to thereby remove relatively short lengths of the traveling strip which contain the imperfect portions.